Add unmarked test

This adds a codegen test which once annotated can check that the optimization is applied in the
case expected. I'm not sure how to annotate it, though, so I've left it unmarked.

Author: JulianKnodt

library/core/src/array/mod.rs

@@ -479,7 +479,7 @@ impl<T, const N: usize> [T; N] {
                 // ManuallyDrop::new(..), and implicitly know that it will be a `dst` variant
                 // from where
                 unsafe {
-                    let v = f(src_dst[i].src.read());
+                    let v = f(src_dst[i].src.assume_init_read());
                     src_dst[i].dst = ManuallyDrop::new(v);
                 }
                 guard.initialized += 1;

src/test/codegen/array-map.rs

@@ -0,0 +1,18 @@
+// compile-flags: -O
+
+#![crate_type = "lib"]
+#![feature(array_map)]
+
+// CHECK-LABEL: @array_inc
+// CHECK-NOT: alloca
+#[no_mangle]
+pub fn array_inc(x: [u8; 1000]) -> [u8; 1000] {
+  x.map(|v| v + 1)
+}
+
+// CHECK-LABEL: @array_inc_cast
+// CHECK: alloca
+#[no_mangle]
+pub fn array_inc_cast(x: [u8; 1000]) -> [u16; 1000] {
+  x.map(|v| v as u16 + 1)
+}